126 Is'ERVOrS TissrE. 



theless, ■^ith that organ in their elementary strnctnre, and to a certain 

 extent also in their relation to the nervous fibres with which the}^ are 

 connected ; and this correspondence becomes even more apparent in 

 the nervous S3'steni of the lower members of the animal series. 



Tlie nerves are divided into the cerebrospinal, and the si/mjmfhefir or 

 (langlionir nerves. The former are distributed principally to the skin, 

 the organs of the senses, and other parts endowed with manifest sensi- 

 bility, and to muscles placed more or less under the control of the will. 

 They are attached in pairs to the cerebro-spinal axis, and like the parts 

 which they supply are, with few exceptions, remarkably symmetrical on 

 the two sides of the body. The sympathetic or ganglionic nerves, on 

 the other hand, are destined chiefly for the viscera and blood-vessels, of 

 which the motions are involuntary, and the natural sensibility is obtuse. 

 They differ also from the cerebro-spinal nerves in having generally a 

 greyish or reddish colour, in their less symmetrical arrangement, and 

 especially in the circumstance that the ganglia connected with them 

 are much more numerous and more generally distributed. Branches of 

 communication pass from the spinal and several of the cerebral nerves 

 at a short distance from their roots, to join the sympathetic, and in 

 these communications the two systems of nerves mutually give and 

 receive nervous fibres ; so that parts supplied by the sympathetic may 

 be also in nervous connection with the cerebro-spinal centre. 



The nervous system is made up of a substance projDcr and peculiar to 

 it, with inclosing membranes, nutrient blood-vessels and supporting 

 connective tissue. The nervous substance has been long distinguished 

 into two kinds, obviously differing from each other in colour, and there- 

 fore named the wliitc, and the grey or cmeritious. 



STRUCTURAL ELEMENTS. 



When subjected to the microscope, the nervous substance is seen to 

 consist of two different structural elements, \\z., fibres and cells. The 

 fibres are found universally in the nervous cords, and tliey also consti- 

 tute the greater ]iart of the nervous centres : the cells on the other 

 hand are confined in a great measure to the cerebro-spinal centre and 

 the ganglia, and do not exist generally in the nerves properly so called, 

 although they liave been found at the terminations of some of the nerves 

 of special sense, and also interposed here and there among the fibres of 

 particular nerves ; they are contained in the grey portion of the brain, 

 spinal cord, and ganglia, which grey substance is in fact made up of 

 these cells intermixed in many parts with fil^res, and with a variable 

 quantity of supporting connective substance. 



The fibres are of two kinds : 1, the irltiie, meih-iJlafed , Uibalar. or darh 

 bordered, and 2, the grey, pale, or non-meduUaled. The former are by 

 far the most abundant ; the latter are found principally in the sympa- 

 thetic nerve, but exist also in the cerebro-spinal nerves. 



The White or Medullated Fibres (fig. 81). — These form 

 the white part of the brain, spinal cord, and nerves. When collected 

 in considerable numbers and seen with reflected light, the mass which 

 they form is white and opaque. Viewed singly, or few together, under 

 the microscope, with transmitted light, they are transparent ; and if 

 quite fresh from a newly killed animal, and unchanged by cold or 

 exposure, they appear as if entirely homogeneous in substance, and are 



